Anna Twarog

The Carpathian Foredeep is an asymmetric foreland basin formed at the front of the Carpathians as a result of the northward thrusting of the orogen. The substrate of the basin comprises rocks from the Epivariscan Platform along with its Permian-Mesozoic cover. The basin is filled with Miocene sediments, which are the primary target for oil and gas exploration in this region. By 2005, more than 70 natural gas fields had been discovered in the Carpathian Foredeep. In the context of hydrocarbon exploration within this basin, eight areas were examined using a surface geochemical method based on analyzing the molecular composition of soil gas samples. Samples were collected from a depth of 1.2 meters using a patented probe and sampling set. The analysis included concentrations of light alkanes and alkenes, as well as hydrogen and carbon dioxide. The laboratory analyses of the molecular composition of the collected samples were performed using gas chromatography with FID and TCD detectors. Generally, hydrogen was not always included in soil gas analyses. When hydrogen was measured, its interpretation was often limited to minor mentions supplementing analysis of anomalous concentrations of alkanes, and alkenes in the context of hydrocarbon accumulation exploration. This study aims to analyze the variations in hydrogen concentrations identified in 2964 soil gas samples from the Polish part of the Carpathian Foredeep, assess the relationships between hydrogen and light hydrocarbons, and interpret the concentration distributions integrated with geological-seismic model. The maximum recorded hydrogen concentration in the samples was 2320 ppm. The average hydrogen concentration in the soil gas across the entire Carpathian Foredeep is about 70 ppm, which is five times higher than the median value. Additionally, hydrogen concentrations show a strong correlation with ethane and the total alkanes C2-C5, with Pearson correlation coefficients of 0.49 and 0.52, respectively. Previous studies have shown that heavier alkanes cannot form in significant quantities in the near-surface zone, suggesting that the presence of these components in soil gas is due to microseepage and migration from deep accumulations or hydrocarbon generation zones. Therefore, the correlation between hydrogen and the total alkanes C2-C5 may indirectly indicate a geological origin for the hydrogen. Noteworthy is the Brzesko-Wojnicz area, where the highest hydrogen concentration in the entire basin was recorded. Elevated concentrations of this gas correlate with zones where the Miocene bedrock lies relatively shallow. Additionally, the Miocene of this area is drilled by numerous wells, which may facilitate hydrogen migration from deeper horizons. The conducted studies have shown that the hydrogen concentrations in soil gas in the Carpathian Foredeep area exhibit significant variability. The research indicates that hydrogen may have a deep origin, likely migrating to the surface from the Miocene substrate along fault zones and fractures. However, at this stage of research, it is difficult to definitively determine whether the mapped zones of anomalous hydrogen concentrations result from its dispersion from deep accumulations. The research project was supported by program “Excellence initiative – research university” IDUB for the AGH University of Krakow (project number 6237).Anna Twaróg1, Henryk Sechman1 1AGH University of Krakow, Faculty of Geology, Geophysics and Environmental Protection, al. A. Mickiewicza 30, 30-059 Krakow, Poland.​